The power or parametric

"Parametric" para = many. Metric= value/variable

Many of the new 3D modellers are ‘parametric modellers. Meaning you assign values to give the model form. You can also go farther and give relationships / equations to your measurements. This can allow geometry to remain the same when a value changes. Supposed you have a hole 1/3 the way along on the side of a block. Your block side will be D1 long by D2 high. You hole can be =D1/3 by D2/2. This way, regardless of what your D1 & D2 values are (the side of your block), the hole will always be 1/3 along, and ½ the height. There is usually a switch to turn on & off the variables, so you can build equations.

With this, you can build something, say a NC, whereas all the dimensions are based off one value, the inner BT diameter.

The more advanced modellers (Fusion 360, Solidworks, etc..) you can build parts form tables; an excel file, whereas all the dimension are listed & read from a spreadsheet. This can help with making equations, and you can see all the variables in one place. You just have to remember which variable is which!

You could build a fin can, and have only 4 variables: BT dia, Fin root dim, Fin tip dim, sweep angle (fin rake), and number of fins around. It would calculate it all for you based on these 4 parameters..

You can then start to really think about your design, and start assigning some rules to all this, to have a really robust design.